fdo#83428: Custom Properties dropped while exporting to docx

Change-Id: Ic289fc446e49fb096cbfad789299736318019e91
Reviewed-on: https://gerrit.libreoffice.org/11265
Reviewed-by: Muthu Subramanian K <muthusuba@gmail.com>
Tested-by: Muthu Subramanian K <muthusuba@gmail.com>